1. Field of the Invention
The present invention relates to plasma reactor apparatus and processes. More specifically, the present invention relates to spiking the voltage to a semiconductor substrate pedestal during a portion of a positive voltage power bias oscillation cycle to reduce or eliminate the detrimental effects of feature charging during the operation of a plasma reactor.
2. State of the Art
Higher performance, lower cost, increased miniaturization of electronic components, and greater density of integrated circuits are ongoing goals of the computer industry. One commonly used technique to increase the density of integrated circuits involves stacking of multiple layers of active and passive components one atop another to allow for multilevel electrical interconnection between devices formed on each of these layers. This multilevel electrical interconnection is generally achieved with a plurality of metal-filled vias (“contacts”) extending through dielectric layers which separate the component layers from one another. These vias are generally formed by etching through each dielectric layer by etching methods known in the industry, such as plasma etching. Plasma etching is also used in the forming of a variety of features for the electronic components of integrated circuits.
In plasma etching, a glow discharge is used to produce reactive species, such as atoms, radicals, and/or ions, from relatively inert gas molecules in a bulk gas, such as a fluorinated gas, such as CF4, CHF3, C2F6, CH2F2, SF6, or other freons, and mixtures thereof, in combination with a carrier gas, such as Ar, He, Ne, Kr, O2, or mixtures thereof. Essentially, a plasma etching process comprises: 1) reactive species are generated in a plasma from the bulk gas, 2) the reactive species diffuse to a surface of a material being etched, 3) the reactive species are absorbed on the surface of the material being etched, 4) a chemical reaction occurs that results in the formation of a volatile by-product, 5) the by-product is desorbed from the surface of the material being etched, and 6) the desorbed by-product diffuses into the bulk gas.
As illustrated in drawing FIG. 4, an apparatus 200 used in the plasma etching process consists of an etching chamber 202 in electrical communication with a first AC power source 204. The etching chamber 202 further includes a pedestal 206 to support a semiconductor substrate 208 and an electrode 212 opposing the pedestal 206. The electrode 212 is in electrical communication with a second AC power source 214. The pedestal 206 may have either an AC (alternating current) bias source or DC (direct current) bias source 216.
In the etching chamber 202, a plasma 222 is maintained by inductively coupling energy from the first power source 204 into the plasma 222, which comprises mobile, positively and negatively charged particles. An electric field, or bias voltage, develops in a sheath layer 224 around the plasma 222, accelerating the electrons and ions (not shown) toward the semiconductor substrate 208 by electrostatic coupling.
To assist with the etching, the potential difference between the plasma 222 and the semiconductor substrate 208 can be modulated by applying an oscillating bias power from the pedestal power bias source 216 to the pedestal 206, as illustrated in drawing FIG. 5 (showing the voltage profiles during such oscillation). During the positive voltage phase 232, the substrate collects electron current from electrons that have enough energy to cross the sheath. The difference between the instantaneous plasma potential and the surface potential defines the sheath potential drop. Since the plasma potential is more positive than the surface potential, this drop has a polarity that retards electron flow. Hence, only electrons with energy larger than this retarding potential are collected by the substrate. During the negative voltage phase 234, positive ions are collected by the substrate. These ions are accelerated by the sheath voltage drop and strike the substrate.
However, it is known that the plasma etching process (as well as ion implantation and other charge beam processes) may cause damage to the semiconductor substrate and to the devices and circuits formed therein or thereon. In particular, electrical charging is a well-known problem which can occur during the plasma processing of semiconductor devices, leading to the degradation of the device performance.
Illustrated in drawing FIG. 6 is the phenomenon of electrical charging on a semiconductor device in the process of a plasma etch. A material layer 244 to be etched is shown layered over a semiconductor substrate 242. A patterned photoresist layer 246 is provided on the material layer 244 for the etching of a via. During the plasma etching process, the patterned photoresist layer 246 and material layer 244 are bombarded with positively charged ions 248 and negatively charged electrons 252 (i.e., the reactive species). This bombardment results in a charge distribution being developed on the patterned photoresist layer 246 and/or the semiconductor substrate 242. This charge distribution is commonly called “feature charging.”
In order for feature charging to occur, the positively charged ions 248 and the negatively charged electrons 252 must become separated from one another. The positively charged ions 248 and negatively charged electrons 252 become separated by virtue of the structure being etched. As the structure (in this example a via 254) is formed by etching, the aspect ratio (height-to-width ratio) becomes greater and greater. During plasma etching, the positively charged ions 248 are accelerated (e.g., as a result of a DC bias at the semiconductor substrate 242) toward the patterned photoresist layer 246 and the material layer 244 in a relatively perpendicular manner, as illustrated by the arrows adjacent positively charged ions 248. The negatively charged electrons 252, however, are less affected by the DC bias at the semiconductor substrate 242 and, thus, move in a more random isotropic manner, as depicted by the arrows adjacent negatively charged electrons 252. This results in an accumulation of a positive charge at a bottom 256 of via 254 because, on average, positively charged ions 248 are more likely to travel vertically toward the substrate 208 than are negatively charged electrons 252. Thus, any structure with a high enough aspect ratio tends to charge more negatively at photoresist layer 246 and an upper portion of the material layer 244 to a distance A (i.e., illustrated with “−” indicia) and more positively at the via bottom 256 and the sidewalls 258 of the via 254 proximate the via bottom 256 (i.e., illustrated with “+” indicia).
As shown in drawing FIG. 7, the positively charged via bottom 256 deflects the positively charged ions 248 away from the via bottom 256 and toward the sidewalls 258 of the via 254, as a result of charge repulsion. The deflection results in an etching of the sidewalls 258 proximate the via bottom 256, known as “notching.” Furthermore, the presence of the positively charged via bottom 256 slows the positively charged ions 248 as they approach the positively charged via bottom 256, thereby reducing etching efficiency.
As shown in drawing FIG. 8, the negatively charged photoresist layer 246 and the upper portion of the material layer 244 deflect the negatively charged electrons 252 away from entering the via 254 or slows the negatively charged electrons 252 as they enter the via 254, both caused by charge repulsion and both of which reduce etching efficiency.
Thus, it can be appreciated that it would be advantageous to develop an apparatus and a process of utilizing a plasma reactor that eliminates or lessens the effect of feature charging, while using inexpensive, commercially available, semiconductor device fabrication components and without requiring complex processing steps.